Cystic fibrosis (CF) is one of the most common life-shortening genetic diseases. It is characterized by pancreatic insufficiency, resulting in malnutrition and growth faltering even in infants diagnosed early through newborn screening. Obstructive lung disease with recurrent respiratory infections is inevitable. Median age of survival has improved to 38 years in 2008, but 20% of deaths still occur before age 20 years. The overall objective of this grant is to assess the effects of exclusive breastfeeding and preschool diet on growth, nutritional status and pulmonary health in CF children through the first 6 years of life and determine if these effects are related to an insufficient essential fatty acid intake from breast milk and variations in gut microbiota. This will be done by expanding the follow-up of a highly productive, strong multi-center prospective observational study being conducted in a network of 5 CF Centers in 4 states (UT, WI, IN and MA) since 2012, referred to as FIRSTage0-2 study. Specific Aims are: 1) to investigate the potential benefits and risks of exclusive breastfeeding (exBF) in 160 children with CF from age 2 to 6 years (referred to as FIRSTage2-6 study) and test the hypotheses that a) the anti-Pseudomonas infection benefit of exBF in the first 2 years of life results in better lung function at age 6 years and b) the early growth-faltering effect of exBF is reduced at 6 years by fortifying human milk and enriching the preschool diet during age 2-6 years; 2) to test the hypothesis that low essential fatty acid concentration of breast milk results from low maternal intake, predisposes exBF infants with CF to essential fatty acid deficiencies, and contributes to poor growth (Breast Milk study); 3) to characterize fecal microbiota using 16S rRNA gene sequencing and testing the hypotheses that a) the gut microbiota differs between breastfed and formula-fed CF infants and is related to the frequency of antibiotic exposure and use of proton pump inhibitor, and b) differences in gut microbiota are related to poor growth/nutritional status and the severity of lung disease (Gut Microbiome study). Research proposed in this grant is timely and highly significant, because as of 2010, NBS for CF is universal in the US as well as in many European countries. However, optimal feeding is unknown in this very early diagnosed population of CF infants. Aim #1 will determine if exclusive breastfeeding is indeed the preferred method of feeding infants with CF as perceived by many CF clinicians. Aim 2 will elucidate the links between maternal diet, breast milk fatty acid content and infant essential fatty acid status in CF, generating the needed evidence to resolve the long-standing controversy of essential fatty acid supplementation for CF infants. Utilizing the wealth of clinical data generated from Aims 1 and 2, Aim 3 will not only advance our understanding of gut microbiome in CF but delineate its impact on clinical outcomes of CF. Accomplishing the aims of this grant will lead to innovative changes in the care of infants with CF.